A garden play room is a garden room with a different set of risks, and most of them come down to two things: what happens when a child runs into glass, and what a heater is within reach of. Neither adds much to the cost if it is decided at specification stage, and both are difficult and expensive to change once the building is glazed and wired.
Glazing in critical locations
Glass low down in a wall, or in and beside a door, sits in what the Approved Documents call a critical location, and it has to be safety glass or otherwise protected. In a room built for children that is not a box to tick, it is the whole point, and it applies to the large low panes that make a play room feel light. Specify it explicitly rather than assuming it is standard.
Heating a child can touch
A freestanding fan heater or an unguarded panel heater in a room used unsupervised is the wrong answer. Fixed heating at high level, underfloor heating, or a wall unit out of reach with a thermostat and a timer all avoid the problem. The heater is also the thing most likely to have something draped over it, so a design that cannot be covered matters.
It is not a bedroom, and that has consequences
Permitted development for outbuildings covers uses incidental to the house, and sleeping accommodation is not one of them. A play room where a child occasionally naps is a play room. A room built and used as a bedroom is a different planning proposition and a different building regulations proposition, with fire escape and warning requirements attached.
Doors, sightlines and getting out
A door a child can open from inside without a key, a route to the house that does not cross anything dangerous, and a window arrangement that lets an adult see in from the house are all worth more than any feature. So is thinking about where the door swings, because a door that opens outwards onto a path is a different hazard from one that opens in.
Questions people ask about garden play room
Does a garden play room need building regulations approval?
A small detached outbuilding with no sleeping accommodation is frequently exempt on floor area and boundary distance grounds, and the electrical installation is notifiable either way. The glazing and the heating are worth specifying to the same standard the Approved Documents would require even where they do not formally apply.
Can children sleep in it during a sleepover?
Occasional use is one thing and a room provided as sleeping accommodation is another, and the distinction matters both for planning and for fire safety. If sleeping is part of the plan, design and declare it that way rather than treating it as an incidental use.
Should the door lock?
From the outside, yes, so the room can be secured when it is not in use. From the inside, no, or at least not with anything a small child can operate and an adult cannot.